link: document sk == NULL case for rtnl_link_alloc_cache()

When sk == NULL, rtnl_link_alloc_cache() returns 0 but the cache
is still empty. We should document this behavior, otherwise it is confusing.

Cc: Thomas Haller <thaller@redhat.com>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>